I've got a Samsung YP-U5JA here and it reproducibly freezes, when it is used the following usual way:

- play an ogg/vorbis title
- press the pause button
- push the back button shortly (you are at the beginning of the title again, playback is still paused)
- move forward to a desired position of the title
- press play and the YP-U5 freezes. It only helps to push the reset button at the back of the player

Remark: This happens only with ogg/vorbis files and not with mp3 or flac. In general the player is working fine for playing ogg/vorbis files except the above case.

Would be interesting if this bug can be reproduced by somebody or reported to Samsung.